Noise and vibration control is a primary concern for the transmission design engineer, and particularly so in the design of a helicopter main transmission. Excessive vibrations generated by transmissions typically result in undesirable noise levels in helicopter cockpits and/or cabins, which causes operator/passenger aural discomfort and/or damage to sensitive on-board instrumentation. Cabin and/or cockpit noise/vibration abatement is a particular concern in helicopters wherein the final stage of reduction gearing of the main transmission comprises one or more bull pinions interacting with a central bull gear.
For example, Sikorsky helicopters of the S-76 series, e.g., S-76A, S-76B, S-76C, have a main transmission that includes three stages of reduction gearing: a first stage for each engine output consisting of helical gearing, an intermediate stage consisting of spiral bevel gearing, and a final reduction stage comprising a central bull gear that intermeshes with right and left hand bull pinions (to combine the inputs of the two engines that provide the motive power for the helicopter). Research has shown that the cockpit and/or cabin noise levels of S-76 helicopters are primarily the result of vibrations originating in the main transmission.
Narrow band Fast Fourier Transform (FFT) analyses, A-weighted octave levels, and overall DBA levels recorded in the cockpits and/or cabins of S-76A, S-76B, and S-76C helicopters indicate that interior noise levels are predominately the result of vibrations occurring at the bull gearing meshing frequency of 778 Hz, as illustrated in FIG. 1. The vibrations produced by the first and second reduction stages of S-76 main transmission gearboxes, i.e., the noise levels generated by the helical and spiral gearing as illustrated in FIG. 1, occur at higher frequencies and typically are not significant relative to the dominant noise levels produced by the fundamental and first few harmonics of the bull gearing meshing vibrations.
The gearbox vibrations resulting from bull gear meshing are transmitted to the helicopter airframe via the transmission housing. The resultant airframe vibrations generate noise in the helicopter cockpit and/or cabin. Abatement of such noise by acoustic treatment of the cockpit and/or cabin interior is generally inefficient, and therefore, effective noise control solutions must be implemented at the noise source, i.e., the main transmission.
To effectively abate such noise, it is necessary to identify the primary causal factor(s) of bull gearing vibrations. The vibrations generated by the aforedescribed helicopter main transmission may be aggravated by meshing between misaligned bull gearing, i.e., the central bull gear and bull pinion(s). Previous efforts to reduce the noise levels generated by intermeshing between misaligned bull pinions and the central bull gear included modifications to provide effective bull gear tip relief. This technique is also referred to as “crowning” of the gear teeth. While such modifications resulted in a modest reduction in bull gearing vibrations, the resultant reduced interior noise levels of S-76 helicopters were adjudged to still present an unacceptable level of aural discomfort.
Yet other methods for reducing noise are described in Hayduk et al. U.S. Pat. No. 5,271,288 wherein tooth topological modifications are employed. Therein the generation of noise is attributed to thermal expansion of and within the transmission gearbox and the differential thermal expansion caused by dissimilar metals/materials (i.e., materials having different coefficients of thermal expansion). As such, testing indicated that the distance between centerlines of the adjacent intermeshing gears grew larger as a result of elevated temperatures. Consequently, material was added to the chordal addenda of the gear teeth (i.e., essentially the top lands) to compensate for the thermally-induced transmission errors. While the patent discloses tooth topological modifications to reduce noise levels, it does not address the root causes of noise generated by intermeshing gears. Consequently, the teachings therein are limited to helicopter gearboxes, and more particularly, to a specific drive train configuration used in a specific helicopter model.
A need exists to more fully identify the design and operating parameters that cause noise induced by the intermeshing gears of a drive train. An effective solution for noise abatement can only be implemented based upon the identification and understanding of the causal parameter(s) of such vibration generation.